Abstract

We have previously shown that local exposure of plants to stress results in a systemic increase in genome instability. Here, we show that UV-C-irradiated plants produce a volatile signal that triggers an increase in genome instability in neighboring nonirradiated Arabidopsis thaliana plants. This volatile signal is interspecific, as UV-C-irradiated Arabidopsis plants transmit genome destabilization to naive tobacco (Nicotiana tabacum) plants and vice versa. We report that plants exposed to the volatile hormones methyl salicylate (MeSA) or methyl jasmonate (MeJA) exhibit a similar level of genome destabilization as UV-C-irradiated plants. We also found that irradiated Arabidopsis plants produce MeSA and MeJA. The analysis of mutants impaired in the synthesis and/or response to salicylic acid (SA) and/or jasmonic acid showed that at least one other volatile compound besides MeSA and MeJA can communicate interplant genome instability. The NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (npr1) mutant, defective in SA signaling, is impaired in both the production and the perception of the volatile signals, demonstrating a key role for NPR1 as a central regulator of genome stability. Finally, various forms of stress resulting in the formation of necrotic lesions also generate a volatile signal that leads to genomic instability.

Role of MeSA and MeJA in Eliciting an Increase in HRF.(A) HRF in wild-type plants after direct spraying with MeSA or MeJA. Three-week-old Arabidopsis plants were sprayed with MeSA or MeJA at various concentrations, ranging from 0.02 to 0.5 mM. Bars show average HRF ± sd of three biological repeats, each one consisting of three pots containing 12 to 20 plants . Asterisks indicate significant difference compared with control mock-sprayed Arabidopsis (P < 0.05).(B) HRF in wild-type Arabidopsis plants after being exposed to a MeSA- or MeJA-enriched environment. Each experimental point consisted of three pots with 12 to 20 plants in each one. Bars represent average HRF ± sd. Asterisks indicate significant difference compared with control (0 mM) unexposed Arabidopsis (P < 0.05).(C) MeSA levels (ng/L) in the headspace of UV-C–irradiated plants. After irradiation with 7.0 J/m2 UV-C, gas samples were taken and MeSA concentration was determined. The data are the average ± sd of four biological repeats.(D) MeJA levels (ng/L) in the headspace of UV-C–irradiated plants. Experimental details are the same as in (C).(E) Time course of volatile production after UV-C irradiation. Bars represent average bystander HRF ± sd of three biological repeats, each one consisting of three pots containing 12 to 20 plants. Asterisks indicate significant difference compared with control mock-sprayed Arabidopsis (P < 0.05). Asterisks indicate a significant difference from control (*P < 0.05 and **P < 0.01).(F) MeSA and MeJA levels (in ng/L) in the headspace of UV-C–irradiated wild-type and npr1 plants. Experimental details are the same as in (C).

Changes in HRF in Hormone Signaling and Hormone Synthesis Mutants.Three-week-old Arabidopsis wild-type (wt) plants (line #15d8) and various mutants (npr1 jar1 ein2 triple mutant [TM]) were either UV-C irradiated to test volatile emission or used as bystander. For (B) and (C), irradiated plants were immediately placed into plastic bags together with nonirradiated plants. Control plants (the wild type or mutants) were placed into sealed bags without irradiation.(A) HRF in UV-C–irradiated wild-type and mutant plants is shown as average (±sd) fold increase over HRF in unexposed plants calculated from four independent experiments, with each experiment consisting of ~100 plants. Asterisks indicate a significant difference from control plants (P < 0.05). Dashed line refers to control, assigned a value of 1.(B) HRF in bystander hormone signaling mutants exposed to volatiles from wild-type irradiated plants. Bars represent average HRF fold increase (compared with control) ± sd of three biological repeats, each one consisting of three pots containing 12 to 20 plants. Asterisks indicate a significant difference from control plants (P < 0.05). Dashed line refers to control, assigned a value of 1.(C) HRF in bystander wild-type plants exposed to volatiles from irradiated hormone signaling or hormone synthesis/accumulation mutants. Bars represent average HRF fold increase (compared with control) ± sd of three biological repeats, each one consisting of three pots containing 12 to 20 plants. Asterisks indicate a significant difference from control plants (P < 0.05). Dashed line refers to control, assigned a value of 1.

The npr1 Mutant Has a Low Basal Level of Homologous Recombination.Distribution of plants (y axis) by number of recombination spots (zero, one, two, three, or more) per plants (x axis). The inset shows the HRF in wild-type (wt) and npr1 plants as an average HRF ± sd of four biological repeats, with each experiment consisting of ~100 plants.

Changes in HRF in Bystander Plants Exposed to Volatiles from Plants Treated with Necrotizing or Non-Necrotizing Types of Stress.(A) HRF in bystander tobacco plants exposed to volatiles from TMV-infected tobacco. Two leaves of 4-week-old tobacco plants (cultivars SR1, susceptible to TMV, or Big Havana [BH], resistant to TMV) were infected with 200 ng of TMV. Infected SR1 or Big Havana plants were placed in a sealed bag side by side with naive tobacco plants of the same cultivar. Each experimental point consisted of three pots with 8 to 12 plants per pot. Bars represent the average HRF ± sd calculated from three biological repeats. Asterisks indicate a significant difference compared with control (ct) (P < 0.05).(B) HRF in Arabidopsis plants exposed to NaCl or to volatiles from NaCl-treated plants (BS). Two-week-old Arabidopsis plants were grown on soil and watered with 250 mM NaCl for 2 d, after which they were placed side by side with naive Arabidopsis plants in sealed plastic bags for 4 d. At day 4 after exposure, bags were opened and HRF was measured 3 d later (7 d after treatment). Each experimental point consisted of three pots with 12 to 20 plants per pot. Bars represent the average HRF ± sd calculated from three biological repeats. Asterisks indicate a significant difference compared with control (P < 0.05).(C) HRF in Arabidopsis plants exposed to zebularine or to volatiles from zebularine-treated plants (BS). Two-week-old Arabidopsis plants grown on soil were sprayed with zebularine and immediately placed side by side with naive Arabidopsis plants in sealed plastic bags for 4 d. At day 4 after exposure, bags were opened and HRF was measured 3 d later (7 d after treatment). Experimental details are the same as in (B). Asterisks indicate a significant difference compared with control (P < 0.05).

UV-C–Induced Necrosis.Plants were irradiated at 20, 25, and 30 d postgermination (dpg). Necrotic lesions were scored 24 h after irradiation and trypan blue staining. Bars show the average number of necrotic lesions ± sd per leaf. Data were obtained from three biological repeats, each one consisting of 20 to 30 leaves. Asterisks indicate a significant difference (P < 0.05) between irradiated (UV) and control (ct) plants. wt, wild type.